The objectives of this study are to study the active site of steroid binding proteins using affinity labels, develop an RIA for progesterone binding globulin, determine the site of and factors controlling the synthesis of PBG, study the function of PBG during gestation, and produce antibodies to the progesterone receptor from hen oviducts. The nature of the amino acids present in the steroid binding site of PBG and chick oviduct progesterone receptors will be determined after inactivation with a variety of affinity labels. In addition, peptides containing the residue modified by affinity labeling will be isolated and their sequence determined to learn more about the nature of the active site. The site of synthesis of PBG will be determined by immunofluorescence and the peroxidase anti-peroxidase (or PAP) technique. The site of synthesis will be confirmed by demonstrating the ability of the tissue to direct synthesis of PBG in organ culture or the ability of polysomes of mRNA isolated from these tissues to direct the synthesis of PBG in cell free protein synthesis systems. These techniques and hormonal manipulations or organ cultures will be utilized to examine the factors regulating the production of PBG. Antibody to progesterone receptors from the hen oviduct will be prepared to aid in active site studies. The function of PBG will be probed using affinity labels and antibodies. These studies should lead to increased understanding of the molecular basis of steroid-protein interactions and of the processes regulating the production of specific proteins synthesized during pregnancy. In addition, the availability of antibodies to progesterone receptors should prove of great aid in the study of hormone action and in the determination of receptor levels in breast carcinoma.